Numerous base stations (or access points; APs) are arranged in a wireless LAN network compliant with IEEE 802.11ax which is a next-generation wireless LAN standard. Specifically, numerous basic service sets (BSS) are arranged in the wireless LAN network. Also, considered in this context is a high-density environment in which numerous terminals or stations (STAs) are interconnected. Multi-user transmission technologies such as multi-user MIMO (Multi-User Multiple-Input Multiple-Output: MU-MIMO) and orthogonal frequency division multiplexing access (OFDMA) allow numerous terminals to simultaneously carry out uplink (UL) transmission to the base station, which is called uplink multi-user (UL-MU) transmission.
Terminals access the wireless medium traditionally using the enhanced distributed channel access (EDCA) method. EDCA defines a set of parameters defined in accordance with access categories (ACs). Backoff counters for the respective ACs are reset when the respective ACs are successfully transmitted. When the channel becomes busy during a transmission wait state, countdown of the backoff counter is suspended while the busy state continues. According to IEEE 802.11ax, multiple terminals are simultaneously given permission to make uplink channel access using a trigger frame transmitted by the base station. Although the period of UL-MU transmission is thought to be handled as a state where the channel is busy, it may be the case that the frame that was backed off for transmission gets transmitted when triggered for UL-MU. In such a scenario, it is necessary to consider how to handle the backoff counters. Further, the problem of fairness arises as a terminal that can carry out UL-MU transmission will have more transmission opportunities using the normal uplink single-user (UL-SU) transmission according to which the terminal itself obtains the access right to access a wireless medium and carry out intended transmission without uplink multiplexing.
Use of two parameters has been proposed as one of the methods of handling the EDCA parameters in UL-MU. According to this proposal, a contention parameter of the terminal that uses UL-MU-MIMO is set to a value which is different than that for a terminal that does not use UL-MU-MIMO. Also, in this context, it has been further proposed that the base station present a pruning value, according to which multiple terminals determines whether to perform contention to get the access right to access a channel as a wild-card resource on the basis of the pruning value.
According to another method, each of terminals to which resource units (RUs) are assigned by the trigger frame uses a different EDCA parameter set. CWmin and AIFSN which are EDCA parameters are incremented relative to a case where UL-MU transmission is not performed, and thereby the base station restricts single-user (SU) transmission using EDCA with regard to these terminals. This will mitigate the contention. However, the problem of fairness remains to be unsolved between the terminal that can carry out UL-MU transmission and a terminal that cannot carry out UL-MU.